1. Field of the Invention
The present invention generally relates to a protection method, a control circuit, and a battery unit. More particularly, the present invention relates to a protection method for preventing batteries from over-discharging and being overcharged, and a control circuit and a battery unit both employed in said protection method.
In recent years, lithium ion (Li+) batteries have been replacing nickel-cadmium (NiCd) batteries and nickel-metal-hydrogen (NiMH) batteries in portable electronic devices such as notebook-type personal computers. Compared with the NiCd batteries and NiMH batteries, the Li+ batteries are lighter but have a larger capacity per unit volume. For this reason, the Li+ batteries are suitable for a device which is preferably light and required to endure continuous use for a long time.
In an over-discharged state, however, the Li+ batteries deteriorate rapidly. Therefore, the Li+ batteries need to be prevented from over-discharging.
2. Description of the Related Art
A battery unit used in a portable electronic device has a plurality of battery cells connected in series. The maximum number of battery cells connected in series in one battery unit is determined by the relationship between the output voltage of the battery unit and a power source voltage supplied from outside at the time of charging. For instance, the output voltage of one NiCd battery cell or one NiMH battery cell is 1.2 V, and the power source voltage supplied at the time of charging is approximately 1.7 V. Since a 16-V output voltage of a battery unit is the most suitable for a general purpose electronic device, the maximum number of NiCd or NiMH battery cells connected in series in the battery unit is 9. On the other hand, the highest possible output voltage of one Li+ battery cell is approximately 4.2 V. Accordingly, the maximum number of Li+ battery cells connected in series in one battery unit is 3.
Unlike a NiCd battery unit and a NiMH battery unit, the Li+ battery unit has a function to protect against short-circuiting inside and outside the Li+ battery unit. This prevents the Li+ battery unit from deteriorating and shortening its life. For instance, if short-circuiting occurs inside or outside the Li+ battery unit, a fuse cuts off an over-discharging current or overcharging current when the discharging current or charging current becomes larger than a predetermined current value. Thus, the LI+ battery unit is prevented from deteriorating and shortening its life.
FIG. 1 is a block diagram of an example battery unit of the prior art, and FIG. 2 is a circuit diagram of a voltage monitor circuit of the example battery unit of the prior art.
In FIGS. 1 and 2, a battery unit 100 comprises battery cells E1, E2, and E3 connected as shown in the figures, a voltage monitor circuit 101, a fuse 102, p-channel FETs 103 and 104, and power supply terminals 105 and 106.
The battery cells E1, E2, and E3 are connected in series. The FET 103 is a charge control FET which functions as a charge control switch. The FET 104 is a discharge control FET which functions as a discharge control switch. The voltage monitor circuit 101 monitors the voltages of the battery cells E1, E2, and E3. In accordance with the respective voltages of the battery cells E1, E2, and E3, the voltage monitor circuit 101 switches on and off the FETs 103 and 104.
As shown in FIG. 2, the voltage monitor circuit 101 comprises an overcharge monitor circuit 101a and an over-discharge monitor circuit 101b. The overcharge monitor circuit 101a monitors whether the battery cells E1, E2, and E3 are in an overcharged state, and switches off the FET 103 when the battery cells are in an overcharged state. The over-discharge monitor circuit 101b monitors whether the battery cells E1, E2, and E3 are in an over-discharged state, and switches off the FET 104 when the battery cells E1, E2, and E3 are in an over-discharged state.
The overcharge monitor circuit 101a comprises comparators 121, 122, and 123, reference power sources e1a, e1b, and e1c, and an OR gate 124.
The comparator 121 compares the voltage of the battery cell E1 with a reference voltage Vref1 generated by the reference power source e1a. If the voltage of the battery cell E1 is higher than the reference voltage Vref1, the comparator 121 outputs “1”. If the voltage of the battery cell E1 is lower than the reference voltage Vref1, the comparator 121 outputs “0”. Here, “1” indicates that the output of a comparator is at the high logic level, and “0” indicates that the output of a comparator is at the low logic level. The comparator 122 compares the voltage of the battery cell E2 with a reference voltage Vref1 generated by the reference power source e1b. If the voltage of the battery cell E2 is higher than the reference voltage Vref1, the comparator 122 outputs “1”. If the voltage of the battery cell E2 is lower than the reference voltage Vref1, the comparator 122 outputs “0”. The comparator 123 compares the voltage of the battery cell E3 with a reference voltage Vref1 generated by the reference power source e1c. If the voltage of the battery cell E3 is higher than the reference voltage Vref1, the comparator 123 outputs “1”. If the voltage of the battery cell E3 is lower than the reference voltage Vref1, the comparator outputs “0”.
The outputs of the comparators 121, 122, and 123 are supplied to the OR gate 124. The OR gate 124 performs an OR operation on the outputs of the comparators 121, 122, and 123, and supplies a result of the OR operation to the gate of the FET 103. If any of the outputs of the comparators 121, 122, and 123 is “1”, i.e., if any of the battery cells E1, E2, and E3 is in an overcharged state and the signal supplied from the OR gate 124 to the gate of the FET 103 is “1”, the FET 103 is switched off so as to prevent overcharge.
The over-discharge monitor circuit 101b comprises comparators 111, 112, and 113, reference power sources e2a, e2b, and e2c, and an OR gate 114.
The comparator 111 compares the voltage of the battery cell E1 with a reference voltage Vref2 generated by the reference power source e2a. If the voltage of the battery cell E1 is higher than the reference voltage Vref2, the comparator 111 outputs “0”. If the-voltage of the battery cell E1 is lower than the reference voltage Vref2, the comparator 111 outputs “1”. The comparator 112 compares the voltage of the battery cell E2 with a reference voltage Vref2 generated by the reference power source e2b. If the voltage of the battery cell E2 is higher than the reference voltage Vref2, the comparator 112 outputs “0”. If the voltage of the battery cell E2 is lower than the reference voltage Vref2, the comparator 112 outputs “1”. The comparator 113 compares the voltage of the battery cell E3 with a reference voltage Vref2 generated by the reference power source e2c. If the voltage of the battery cell E3 is higher than the reference voltage Vref2, the comparator 113 outputs “0”. If the voltage of the battery cell E3 is lower than the reference voltage Vref2, the comparator 113 outputs “1”.
The outputs of the comparators 111, 112, and 113 are supplied to the OR gate 114. The OR gate 114 performs an OR operation on the outputs of the comparators 111, 112, and 113, and supplies a result of the OR operation to the gate of the FET 104. If any of the outputs of the comparators 111, 112, and 113 is “1”, i.e., if any of the battery cells E1, E2, and E3 is in an over-discharged state and the signal supplied from the OR gate 114 to the gate of the FET 104 is “1”, the FET 104 is switched off so as to prevent over-discharge.
When a current larger than a certain current value flows, the fuse 102 fuses and cuts off the current. By doing so, the fuse 102 serves as a part of a double protection circuit in a case where the voltage monitor circuit 100 does not properly cut off the large current or the FETs 103 and 104 do not properly function to cut off the large current due to some trouble such as short-circuiting.
The power supply terminals 105 and 106 are connected to an electronic device 130, as shown in FIG. 1. The electronic device 130 comprises a power source circuit 131 and a device main body 132. The power source circuit 131 converts a d.c. voltage supplied from the battery unit 100 to a d.c. voltage to be used in the device main body 132.
At the time of shipping, the battery unit 100 is connected to the electronic device 130. The battery unit 100 may be fixed to the electronic device 130 with screws. If the battery unit 100 and the electronic device 130 are packed separately in such a case, the package becomes large, and a large amount of cushioning material is required. Moreover, after unpacking, the user has to take the trouble to screw the battery unit 100 to the electronic device 130.
In a case of an electronic device having built-in dry batteries, an insulating sheet is inserted between the dry batteries and the electrodes of the electronic device. The user normally removes the insulating sheet when he/she starts using the electronic device. By removing the insulating sheet, the dry batteries and the electronic device are connected, and electric power is supplied from the dry batteries to the electronic device. Compared with the dry batteries, however, the battery unit 100 has more connection pins for connection with the electronic device 130. Also, the connection connector of the battery unit 100 has a more complicated structure. For these reasons, an insulating sheet cannot be inserted between the battery unit 100 and the electronic device 130, and, at the time of shipping, the battery unit 100 is already mounted on the electronic device 130, as shown in FIG. 1.
The battery unit 100 shown in FIG. 1 remains connected to the power source circuit 131 even when the power switch of the electronic device 130 is turned off. The power source circuit 131 is formed by a DC-DC converter, and consumes electric current even when the output is cut off. The voltage monitor circuit 101 of the battery unit 100 also constantly consumes a small amount of electric current. Because of this, after the shipping of the electronic device 130, the battery cells E1, E2, and E3 of the battery unit 100 are consumed. If the battery unit 100 is in an over-discharged state due to the consumption of the battery cells E1, E2, and E3, the FET 104 is switched off, and the battery cells E1, E2, and E3 are disconnected from the electronic device 130. If the electronic device 130 is left unpacked for an even longer period of time, the battery cells E1, E2, and E3 might over-discharge due to current consumed by the voltage monitor circuit 101.